Generally, in this type of the instrument, a needle fixed to a driving end of an instrument movement is rotated corresponding to an input signal and a measurement value is indicated by a correspondence to an indicator panel marked with a numeral or graduation indicating a quantity to be measured.
Particularly in the stepping motor, stepping operation of its magnet rotor is determined depending on the number of teeth of a comb tooth yoke and its formation pitch. To ensure a smooth rotary operation, the number of teeth is increased so as to make fine comb tooth pitch or so-called micro step driving is executed by driving signals. These styles are selected by an allowable size of the stepping motor depending on use condition or cost including the driving circuit.
Further, such a stepping motor is desired to be of small size regardless of any application purpose. Because a so-called PM type stepping motor is simply structured, the stepping motor has been easy to use because of improvement of the rotor magnet and comb tooth yoke.
Against digitalization (microcomputer control) of a processing circuit, a stepping motor in which pulse signal control is carried out has attracted public attention as a movement of an indication instrument in which an indicated value is read out by a needle by correspondence to a graduation on the character panel. For example, this is applicable for a traveling speed indicator or engine rotation meter of a vehicle and further by A/D processing on detection signals, it can be used also for a fuel gauge and thermometer. Currently, various proposals have been made for practical application as disclosed in Unexamined Published Japanese Patent Application No. 61-129575, Unexamined Published Japanese Patent Application No. 1-223312.
In a case when the stepping motor is used as a movement of an instrument like this one, unlike a vector follow-up type in which a rotary magnet follows in a direction of synthesis vector of a coil magnetic field like a cross coil type instrument, in many cases, a change amount of each predetermined frequency of input signal is obtained and a stepping drive is executed by each change amount. Because the magnetic pole of the rotary magnet is driven for each pitch of the comb tooth yoke or by micro step processing between the pitches, and therefore a general structure employed is that a position of the rotary magnet when power switch is ON is assumed to be an initial value (in an indication instrument, start point initialization processing is carried out by forcibly returning to a zero position on the character panel) and then the stepping drive is executed by each incremental or decremental amount of the input signal from this start point.
Therefore, due to mechanical influences upon the rotary magnet by external vibration on an instrument device, the angular position is deviated from that controlled by stepping drive from a start point, namely deviated from a proper positional relation between the comb tooth yoke and magnetic pole of the rotary magnet, so that the angle may deviate by one pitch. If such an angular deviation or so-called step-out occurs, the proper angular position is never indicated until start point is initialized by turning on the power, so that an angle including an error due to the step-out is indicated.
This kind of step-out phenomenon occurs when the rotary magnet does not follow up a change in input signal as well as due to mechanical deflection by external vibration or the like. That is, if a stepping drive torque relative to the rotary magnet of the stepping motor cannot be secured because of structural reason, driving current limitation or the like or if a change in input signal relative to a limit of such a driving torque or angular speed is too large, the rotary magnet cannot follow up so that step-out occurs repeatedly and finally a large angular error is caused.
Particularly in the stepping motor, the stepping motion corresponding to tooth pitch of the comb tooth yoke is basically employed and smoothness is obtained by correcting the waveform of the drive signals. However, such a smoothing processing is only smoothing between the tooth pitches of the comb tooth yoke, and therefore the step-out due to pitch-over still occurs. Thus, when the measurement quantity changes rapidly so that an accompanying change in input signal is rapid, exceeding a follow-up limit of the rotary magnet, the rotary magnet of the stepping motor cannot follow up so that an accurate indication characteristic as an indication instrument cannot be obtained.
Accordingly, it is an object of the present invention to execute securely follow-up of the rotary magnet and make it difficult to induce step-out by delaying the updating signal of the stepping motor if, with respect to indication angle data obtained by a processing means which calculates measurement data and converts to indication angle signal, a change in the data corresponding to a change in measurement quantity is large.